Fix Python from_dict() round-trip for optional fields with schema defaults

[stephentoub]

Fixes #1139.
Fixes #1140.
Fixes #1141.

Why

from_dict(to_dict(x)) was not the identity for three generated Python dataclasses whenever the affected optional field was None. The dataclass declares the field as T | None = None and to_dict() omits it when None, but from_dict() was substituting the JSON-Schema default value when the key was missing, silently mutating unset fields:

• SessionTaskCompleteData.summary: None → ""
• PermissionPromptRequest.action: None → PermissionPromptRequestMemoryAction.STORE
• PermissionRequest.action: None → PermissionRequestMemoryAction.STORE

Any code that round-trips events (caching, replay, audit logs, dedup) or pattern-matches on None to mean "absent" was getting the wrong answer end-to-end. The bug was reproducible on main as of today.

What changed

Root cause is in scripts/codegen/python.ts: for optional fields with a schema default, the codegen was emitting obj.get("key", "<default>"), then passing that into from_union([from_none, parse_X], ...), which happily parsed the default and returned it. The dataclass default and the wire default were inconsistent, so the round-trip broke.

• Dropped the defaultLiteral mechanism from both emitPyClass and emitPyFlatDiscriminatedUnion. from_dict() now uniformly emits obj.get(key), so missing/null keys flow through from_none and land at the dataclass-declared None. Removed the now-unused toPythonLiteral helper.
• Regenerated python/copilot/generated/session_events.py via npm run generate:python. Exactly three call sites changed, all matching the bug reports.
• Flipped the two codegen-level assertions in nodejs/test/python-codegen.test.ts that previously locked in the buggy output, and added negative assertions to prevent regression at the generator level.
• Replaced test_schema_defaults_are_applied_for_missing_optional_fields in python/test_event_forward_compatibility.py (which asserted the bug as expected behavior) with two regression tests covering missing-key parsing, explicit-null parsing, and the full from_dict(to_dict(x)) round-trip for all three affected classes.

Surface area / non-obvious notes

• Other languages (Go, .NET, Rust, TypeScript) are not affected. Their generators never read propSchema.default to construct deserialization fallbacks; they use real nullable types end-to-end.
• Grep on the post-fix generated session_events.py and rpc.py confirms zero remaining obj.get("...", "...") patterns, so no other class was silently relying on baked-in schema defaults.
• The test_schema_defaults_are_applied_for_missing_optional_fields test was the only consumer in the repo of the old behavior. Nothing in runtime code depended on request.action == STORE or task_complete.summary == "" for unset fields.
• JSON Schema default is an annotation, not validation behavior, so applying it only during deserialization while ignoring it in the dataclass field and in to_dict() was internally inconsistent. This change makes Python deserialization symmetric with serialization and with the in-memory default.

Validation

• python -m pytest test_event_forward_compatibility.py -> 10/10 pass (including new regression tests).
• Broader Python suite: 136/136 non-E2E tests pass.
• npx vitest run on python-codegen.test.ts and other non-CLI test files: pass.
• ruff check, ruff format --check, ty check copilot, tsc --noEmit, eslint: clean.
• Manual reproduction of each issue's repro script on unmodified main (changes stashed) confirms the bug; with the fix applied, all three round-trip with equal? True.

Independent review by the code-review agent on claude-opus-4.7-xhigh found no significant issues and confirmed both codegen paths and the test coverage.

[Copilot]

Pull request overview

Fixes Python generated event-model deserialization so that missing optional fields no longer silently substitute JSON-Schema default values, restoring from_dict(to_dict(x)) identity when those fields are None (addressing #1139, #1140, #1141).

Changes:

• Updated Python codegen to stop emitting obj.get(key, <schema default>) fallbacks during from_dict() generation and always use obj.get(key).
• Regenerated python/copilot/generated/session_events.py to remove the three problematic defaulted obj.get(...) call sites.
• Updated generator-level (Node) and Python regression tests to lock in correct missing-key / explicit-null / round-trip behavior.

Show a summary per file

File	Description
scripts/codegen/python.ts	Removes schema-default fallback emission in from_dict() generation (both class and flat discriminated-union paths).
python/copilot/generated/session_events.py	Regenerated output removing defaulted obj.get(..., default) for the affected optional fields.
nodejs/test/python-codegen.test.ts	Adjusts assertions to expect obj.get(key) and adds negative assertions preventing regression.
python/test_event_forward_compatibility.py	Replaces the prior test that encoded the buggy behavior with regression tests for missing/null parsing and from_dict(to_dict(x)) round-trip.

Copilot's findings

• Files reviewed: 3/4 changed files
• Comments generated: 0

[stephentoub]

@brettcannon would you mind looking at this? Is it the right fix?

[github-actions]

Cross-SDK Consistency Review ✅

This PR fixes a Python-specific codegen bug and maintains cross-SDK consistency.

Analysis:

The changes touch only:

• scripts/codegen/python.ts — removes the defaultLiteral mechanism that baked JSON Schema default values into Python deserialization
• python/copilot/generated/session_events.py — regenerated output (3 call sites changed)
• nodejs/test/python-codegen.test.ts — updated codegen test assertions
• python/test_event_forward_compatibility.py — regression tests

Cross-language impact: None needed. The PR description correctly notes that Go, .NET, Rust, and TypeScript generators were never affected — they use real nullable types end-to-end and don't synthesize default fallbacks during deserialization. I verified that no other codegen script in scripts/codegen/ reads propSchema.default to construct deserialization fallbacks, and the post-fix session_events.py contains no remaining obj.get("key", default) patterns.

Consistency result: The fix actually brings Python into better alignment with the other SDKs — missing optional fields now uniformly return None rather than a schema-default value, matching the behavior users would expect from Go's nil, .NET's null, and TypeScript's undefined/null for unset optional fields.

No changes needed in other SDK implementations.

Generated by SDK Consistency Review Agent for issue #1313 · ● 146.1K · ◷